W. Eckstein

Tridyn-binary collision simulation of atomic collisions and dynamic composition changes in solids

Abstract A computer program is described which treats the slowing down of ions in solids, and the associated formation of recoil atom cascades in the binary collision approximation. For multicomponent media, it takes into account dynamic alterations of the local composition which arise from the deposition of implanted ions and the collisional transport of target atoms. In this way, phenomena can be simulated which occur during high-fluence ion bombardment, such as implantation profiles, atomic mixing and recoil implantation, and composition changes due to preferential sputtering.

Trapping of deuterium implanted in carbon and silicon: A calibration for particle-energy measurements in the plasma boundary of tokamaks

Measurements have been made of the number of deuterons retained in carbon and silicon as a function of fluence, for incident energies between 50 eV and 1 keV. Three independent techniques were used for measuring the retained D: (1) by probing the trapped deuterons with 790 kV 3He+, and counting the total proton yield from the D(3He, H)4He nuclear reaction, (2) by measuring the re-emitted deuterium during and after implantation using mass spectrometry, (3) by thermal desorption mass spectrometry. Initially, the amount of trapped hydrogen increases proportionally to the fluence, while at high doses a saturation value is reached. The quantity of hydrogen trapped at saturation as a function of particle energy follows a power law. The data have particular significance in the estimate of the mean energy of particles to the near-wall region in tokamaks by observing the build-up of trapped hydrogen in a carbon probe during cumulative discharges. As an aid to the interpretation of such data, a multi-energy implant simulating a Maxwellian energy distribution has been made, and the trapping characteristics have been investigated as a function of incident ion fluence.

Data Compendium for Plasma-Surface Interactions

A review of particle-solid processes pertinent to modelling plasma-wall interactions is presented, and sets of recommended data are given. Analytic formulas are used where possible; otherwise, data are presented in the form of tables and graphs. The incident particles considered are e−, H, D, T, He, C, O, and selfions. The materials include the metals aluminum, beryllium, copper, molybdenum, stainless steel, titanium, and tungsten and the nonmetals carbon and TiC. The processes covered are light ion reflection, hydrogen and helium trapping and detrapping, desorption, evaporation, sputtering, chemical effects in sputtering, blistering caused by implantation of helium and hydrogen, secondary electron emission by electrons and particles, and arcing.

Implantation Profiles of Low-Energy Helium in Niobium and the Blistering Mechanism

The depth profiles of 1.5–15‐keV 3He ions implanted into a Nb single crystal at doses of 5×1016–7×1018/cm2 have been measured using the 3He  (d,p)  4He reaction. A comparison of the results with theoretical predictions for the range and the damage distribution of 3He in amorphous material shows reasonable agreement. Furthermore, the Deckeldicke (i.e., thickness of the covers of the blisters) was determined by Rutherford backscattering in double alignment. The results indicate that stress release rather than explosion of gas bubbles is the dominant mechanism in blister formation.

Round Robin computer simulation of ejection probability in sputtering

Abstract We have studied the ejection of a copper atom through a planar copper surface as a function of recoil velocity and depth of origin. Results were obtained from six molecular dynamics codes, four binary collision lattice simulation codes, and eight Monte Carlo codes. Most results were found with a Born-Mayer interaction potential between the atoms with Gibson 2 parameters and a planar surface barrier, but variations on this standard were allowed for, as well as differences in the adopted cutoff radius for the interaction potential, electronic stopping, and target temperature. Large differences were found between the predictions of the various codes, but the cause of these differences could be determined in most cases. A fairly clear picture emerges from all three types of codes for the depth range and the angular range for ejection at energies relevant to sputter ejection, although a quantitative discussion would have to include an analysis of replacement collision events which has been left out here.

THE INFLUENCE OF SURFACE ROUGHNESS ON THE ANGULAR DEPENDENCE OF THE SPUTTER YIELD

Abstract A new approach to study the influence of target surface roughness on sputter yields is explored. The method is based on the experimental determination of the surface topography and the subsequent evaluation of a distribution of local angles of incidence for the incident ions. This distribution is used as input to the Monte Carlo program TRIM.SP for the calculation of the sputter yield of the target with the rough surface. Additionally, the redeposition of sputtered target atoms on the rough surface is calculated. The resulting net sputter yields determined with this procedure are in satisfactory agreement with experimental data.

The Charge States of He and Ne Backscattered from Ni in the Energy Range of 1.5_keV to 15_keV

Abstract Polycrystalline Ni was bombarded by He and Ne ions with energies of 1.5–15 keV. Energy distributions of backscattered ions and neutral atoms were measured and the charged fractions were calculated as a function of emergent energies. Particles backscattered from the surface have larger charged fractions than those from deeper layers. For the latter the charged fraction increases almost linearly with energy.

Reflection of Heavy Ions

The backscattering of heavy ions from surfaces is investigated by computer simulation (TRIM.SP). The particle and energy reflection coefficients are shown to scale with the ratio of target mass to ion mass as well as with the reduced energy ε for ε>0.02 and for normal incidence. At lower energies not only the scaling breaks down but also the effect of a chemical binding influences the reflection coefficient. The calculated data include also the dependence of the reflection coefficients on the angle of incidence as well as angular and energy distributions. Simulated results are in reasonable agreement with those from analytical theory and experiment.

Rutherford backscattering from layered structures beyond the single scattering model

Rutherford backscattering spectra have been calculated with a parallel version of TRIM.SP for 0.5 and 1 MeV 4He normally on a target consisting of about 100 nm Au on Si. The calculated energy distributions are compared with experimental distributions and with those calculated with SIMNRA including double scattering. The contributions of plural scattering on the distributions are determined.

Positive charge fractions of H, D, and He backscattered from solid surfaces

Positive charge fractions of H, D, and He with energies between 3–20 keV backscattered from polycrystalline Au, W, Mo, Ni, Ti, Si, C, WO3, TiH2 and Si3N4 surfaces were measured. For H and D no dependence on the penetration depth of the emerging particle has been observed. He shows higher charge fractions for particles backscattered from the surface compared to those emerging from the bulk. In the case of H and D compound targets yield higher charge fractions compared to the respective elemental targets. These experimental results are compared with the available results of other authors and discussed in the light of existing theoretical models.